Progress in understanding functional somatic symptoms and syndromes in light of the <scp>ICD</scp>‐11 and <scp>DSM</scp>‐5
Authors: Francis Creed
Journal: World Psychiatry (2023)

It is over a decade since the new diagnosis of Somatic Symptom Disorder (SSD) was introduced in the DSM-5, and Bodily Distress Disorder was proposed for inclusion in the ICD-11. These new diagnoses were introduced to move away from the terms "somatoform" and "somatization", which were thought to be unhelpful to patients and doctors. It was also thought necessary to define these disorders in a positive way rather than as "medically unexplained" symptoms, an unsatisfactory term as doctors frequently disagree about whether or not a symptom is explained by a medical disorder. The new classifications aimed to rely more on the presence of definite psychological and behavioral features. The mode of working of the DSM-5 and ICD-11 relevant committees differed considerably. The DSM-5 group held monthly meetings by conference call and annual face-to-face meetings over a period of five years. This process was described as "not a dry scholarly debate but one marked by disputation and passion, yet thankfully also informed by data"1. By contrast, the ICD-11 group held very few formal meetings, with most work done by editing drafts of the diagnostic requirements. Bodily Distress Disorder appeared in a descriptive form, whereas the DSM-5 developed specific criteria for SSD that were more readily operationalized. This may partly explain the remarkable research activity concerning SSD over the last decade. Empirical work has found evidence of good reliability, validity and clinical utility of SSD, which were improvements on previous diagnoses2. No such body of literature has been published yet concerning Bodily Distress Disorder. The prevalence of SSD in the general population has yet to be established. This requires a new measurement tool to detect the condition accurately in large surveys. A questionnaire has been developed to measure the cognitive, affective and behavioral aspects of the B criteria (excessive thoughts, feelings or behaviors related to somatic symptoms, with disproportionate thoughts about their seriousness; high health anxiety or excessive time and energy devoted to them). This work has shown, for example, that some people with SSD spend up to four hours per day concerned with somatic symptoms3. Interestingly, time dedicated to somatic symptoms proved to be an independent predictor of physical health-related quality of life and health care utilization. The other independent predictors were number of somatic symptoms, the other SSD B criteria, anxiety/depression, and age3. It was feared that SSD would be overinclusive, because criterion A requires only one distressing or disruptive somatic symptom. The evidence to date suggests otherwise; the B criteria limit the number of patients who are diagnosed with SSD. There is some evidence that the SSD criteria are associated with higher symptom severity and more impaired physical functioning than the corresponding DSM-IV criteria for somatoform disorders. The B criteria chosen for the definition of SSD have been criticized as "not a reliable guide" by the EUROSOMA group, which has proposed a new classification of "functional somatic disorders" based solely on somatic symptoms and not at all on their cause4. These disorders are conceptualized as "occupying a neutral space within disease classifications, favouring neither somatic disease aetiology, nor mental disorder"4. This suggestion is supported by recent work showing that a high number of somatic symptoms should not be regarded primarily as a psychiatric problem; stressful life events, general medical illnesses and neuroticism are stronger predictors than psychiatric disorders5. The main difficulty with the "functional somatic disorders" classification is that it conflates two distinct, but overlapping, sets of disorders: one is characterized by a high number of troublesome somatic symptoms, and the other by a cluster of specific symptoms which fulfil the diagnostic criteria for one or more functional somatic syndromes (e.g., irritable bowel syndrome, chronic fatigue syndrome, fibromyalgia). Most people with functional somatic syndromes do not have symptoms which fulfil the criteria for SSD. Even in severe irritable bowel syndrome, only about half of people report a high number of somatic symptoms, and they are those who benefit most from psychotherapy or antidepressants. About half of new onsets of self-reported fibromyalgia occur in participants who have a low somatic symptom count; those with and without multiple somatic symptoms appear to have different risk factors6. Recent research has emphasized differences in symptom perception in the different disorders. The findings are consistent with predictive coding theory, which highlights a decoupling of somatosensory input and the perception of body sensations7. For example, interoceptive inaccuracy appears to be a feature of the functional somatic syndromes, whereas a more liberal response bias has been observed in SSD7. Such research may lead to improved classifications in the future, and is important in developing specific treatments. Several recent epidemiological studies suggest that the risk factors for functional somatic syndromes can be best understood by examining specific syndromes, or even subgroups of them, rather than lumping them together. Patients with fibromyalgia have been found to carry substantial genetic risks for pain syndromes and internalizing, autoimmune and sleep disorders; this pattern was quite different from that seen in chronic fatigue syndrome and irritable bowel syndrome8. Another study found that the predictors of self-reported irritable bowel syndrome, chronic fatigue syndrome and fibromyalgia were mostly syndrome-specific, with only four predictors common to all three syndromes9. In that study, psychiatric disorder was a predictor of irritable bowel syndrome, but not of the other two syndromes. Further analysis suggests that there is a subgroup of self-reported irritable bowel syndrome preceded by psychiatric disorder, which appears to have somewhat different risk factors from the remainder. Examining the mechanisms of symptom development in these subgroups may be more rewarding than doing so in the entire syndromes. In conclusion, the new diagnostic entities introduced by the DSM-5 and ICD-11 (SSD and Bodily Distress Disorder) have successfully moved away from definitions based on "medically unexplained symptoms". The inclusion of specific psychological and behavioral features appears to be useful for both clinical and research purposes. The move from cross-sectional clinical studies to population-based cohort ones has been particularly informative concerning risk factors for this group of disorders, confirming that SSD and functional somatic syndromes are different sets of disorders, and that there are differences in risk factors both between and within functional somatic syndromes. Smaller psychological and physiological studies are becoming more productive now that they are focusing on specific patient groups. It is reasonable to expect that our knowledge of somatic symptoms and syndromes will develop greatly over the next decade.